For example, in elevator hoisting machines, the braking device generally used is a machine brake which mechanically engages the rotating part of the hoisting machine. The machine brake may be constructed e.g. as a drum brake or a disc brake. The machine brake is activated by interrupting the supply of electric current to the electromagnet of the brake. The machine brake usually has springs which, when the brake is activated, force an armature part provided with a brake pad against the braking surface of the rotating part to brake the motion of the rotating part of the hoisting machine.
The brake is released by supplying current to the electromagnet of the brake. Release of the brake is effected as the electromagnet pulls the brake pad off the braking surface of the rotating part of the hoisting machine by resisting the pushing force produced by the springs. During elevator operation, the electromagnet remains connected to the power supply, so the brake is in the released state and the elevator car can move up or down in the elevator shaft. The elevator brake may be e.g. so implemented that the same hoisting machine comprises two or more machine brakes.
When the current through the electromagnet is falling, the force applied by the spring finally exceeds the force of attraction of the electromagnet, and the brake is activated. As a result of the imbalance between the forces, the brake pad strikes against the braking surface of the rotating part of the machine. When the brake is released, the electromagnet again applies to the armature part a force opposing the spring force. When the force applied by the electromagnet to the armature part reaches a level exceeding the spring force, the air gap between the frame part and the armature part is closed and the armature part strikes against the frame part.
The impact occurring between the metallic brake parts upon activation or release of the brake may produce disturbing noise. There have been attempts to eliminate the noise problem by adding e.g. in the air gap between the frame part and the armature part a separate damper which in the released state of the brake prevents direct contact between the metal surfaces of the frame part and the armature part. The damper can be made from elastic material, such as rubber or polyurethane; the damper can also be implemented as a spring designed for this purpose, such as a helical spring or diaphragm spring.
The air gap between the frame part and the armature part may vary due e.g. to the manufacturing tolerances of the damper. The force of attraction of the electromagnet is reduced as the airgap is increased, which means that releasing the brake/keeping the brake in the released state requires an increased amount of electric current. Generally speaking, power losses in the coil of the electromagnet are increased with the increase of the airgap, and as the power losses are increased, the operating temperature of the brake also rises.
The damping power produced by the damper may also decline in the course of the service life of the brake; for example, the damping power of a damper made of polyurethane may gradually fall due to a high temperature. Again, the damping power of a spring may decline due to fatigue, among other things. Decline of the damping power involves a deterioration of the functional capacity of the damper, i.e. its ability to dampen the noise of the brake.